Method of and apparatus for influencing the rate of feedwater supply to forced flow steam generators



Aug. 23, 1966 F. LAUBL: 3, ,9

METHOD OF AND APPARATUS FOR INFLUENCING THE RATE OF FEEDWATER SUPPLY ToFORCED mow STEAM GENERATORS Filed June 29, 1964 5 Sheets-Sheet 1Inventor Fkl 7' Z 4 H051.

23, 1965 F. LAusu 3,267,912

METHOD OF AND APPARATUS FOR INFLUENCING THE RATE OF FEEDWATER SUPPLY TOFORCED FLOW STEAM GENERATORS Filed June 29, 1964 5 Sheets-Sheet 2jm/gmon' fie/r2 z A (/5;

By 1516 Aztar ey Aug. 23, 1966 F. LiiUBLI METHOD OF AND APPARATUS FORINFLUENCING THE RATE OF FEEDWATER SUPPLY TO FORCED FLOW STEAM GENERATORSFiled June 29, 1954 5 Sheets-Sheet 5 A zforney r' 3,Z57,l2 [C6 PatentedAugust 23, 1966 3,267,912 METHGD OF AND APPARATUS FOR INFLUENC- ING THERATE OF FEEDWATER SUPPLY TO FURCED FLUW STEAM GENERATGRS Fritz Liiubii,Winterthur, Switzerland, assignor to Sulzer Brothers Limited,Wintcrthur, Switzerland, a corporation of Switzerland Filed June 29,1964, Ser. No. 378,775 Claims priority, application Switzerland, July 4,1963, 8,332/63 10 Claims. (Cl. 122-451) The present invention relates toa method of and apparatus for influencing the rate of feedwater supplyto forced flow steam generators having water separators. The referenceinput signal in the aforedescribed method is the temperature of a tubeof the evaporating section of the forced flow steam generator which tubeis so operated as to produce slightly superheated steam.

It is known to measure the temperature near the outlet end of at leastone of a plurality of tubes of a forced flow steam generator which tubesform the evaporator of the steam generator and to influence the rate offeedwater supply to the steam generator according to said temperature.The tube or tubes of the evaporator at the outlet of which thetemperature is measured are operated to produce slightly superheatedsteam whereas the outlet ends of the other tubes of the evaporatorgenerally conduct saturated steam. A temperature change at the outlet ofone of the utbes which is operated to produce slightly superheated steamcorresponds to a change of the moisture content of the operating mediumat the outlets of the other tubes of the evaporator which outlets areupstream of a water separator. In order to obtain a constant averagemoisture content and, consequently, a predetermined rate of blowdown theset point for the temperature of the steam at the outlet of the tube ortubes which is or are operated to produce slightly superheated steam isconventionally controlled in response to the load on the steamgenerator.

Ascertaining the relation between the load and the set point for thetemperature of the steam leaving the tube of the evaporator producingslightly superheated steam is difiicult and uncertain. Scaling andfouling of the tubes have a disturbing effect. Disturbances due topressure variations at the steam-consuming end of the plant cannot becorrected.

It is an object of the present invention to provide a method andapparatus which avoid the atoredescribed disadvantages of conventionalarrangements.

In the method according to the invention the set point for thetemperature of the steam leaving a tube of the evaporator which tube isoperated to produce slightly superheated steam is influenced by a signalwhich depends at least on the steam pressure measured in theneighborhood of a water separator which is arranged immediatelydownstream of the evaporator.

In a modification of the invention the set point is additionallyinfluenced by a signal which depends at least on one temperature whichis measured in the evaporator upstream of the location where evaporationof the operating medium begins.

In the method according to the invention disturbing influences which arecaused by changes of the temperature upstream or within the zone whereinthe operating medium is evaporated are used to advantage for forming theset point for the temperature at the outlet of the evaporator tube whichis operated to produce slightly superheated steam and for therebymaintaining a constant moisture content of the steam in the waterseparator. In the method according to the invention the aforesaid setpoint is reduced upon increasing temperature upsteam of or within theevaporating Zone.

In a forced flow steam generator wherein the rate of feedwater supply isinfluenced according to the aforedescribed method a pressure-sensitivedevice is arranged in the neighborhood of the water separator and atemperature-sensitive device is placed at the outlet of a tube of theevaporator which is operated to produce slightly superheated steam. Bothdevices are operatively connected to a regulator for influencing therate of feedwater supply to the evaporator.

In a further development of the invention at least onetemperature-sensitive device is additionally provided at a location inthe evaporator upstream of the point where evaporation of the operatingmedium begins. This additional temperature-sensitive device influencesthe set point for the temperature of the steam at the outlet of the tubeof the evaporator which is operated to produce slightly superheatedsteam.

In another embodiment of the invention a function transmitter is usedfor forming the set point for the outlet temperature of at least one ofthe evaporator tubes which is operated to produce slightly superheatedsteam. The input side of this transmitter is operatively connected to apressure-sensitive device placed in the neighborhood of the waterseparator and is also operatively connected to a temperature sensitivedevice which is placed upstream of the location where evaporation of theoperating medium in the evaporator begins.

The regulator to which the pressure-sensitive device located in theneighborhood of the water separator and the temperature-sensitive devicemeasuring the tempera ture at the outlet of an evaporating tube areoperatively connected preferably has a proportional-integral-derivativecharacter. The output signal of this regulator may be used as set pointsignal for an additional control circuit which controls the rate offeedwater supply of the forced flow steam generator.

The novel features which are considered characteristic of the inventionare set forth with particularity in the appended claims. The inventionitself, however, and additional objects and advantages thereof will bestbe understood from the following description of embodiments thereof whenread in connection with the accompanying drawing wherein:

FIG. 1 is a diagrammatic part-sectional illustration of a controlarrangement according to the invention and shows parts of a forced fiowsteam generator essential for the arrangement according to theinvention.

FIG. 2 is a diagram illustrating a modified control arrangementaccording to the invention.

FIG. 3 is a diagrammaic part-sectional illustration of a furtherembodiment of a control arrangement accord ing to the invention.

FIG. 1 is a schematic illustration of a portion of a forced flow steamgenerator into which water is fed by afeed pump 1 through a feed valve2. The generator comprises an economizer 3, an evaporator 4, a collector5 at the outlet of the evaporator, a water separator 6, a preliminarysuperheater 7, and a final superheater 8 wherefrom steam is conducted toa consumer, not shown, through a pipe 9;. The evaporator 4 comprises aplurality of tubes arranged in parallel relation with respect to theflow of operating medium therethrough. Flow of oper-at ing mediumthrough one of the evaporating tubes, which is designated by numeral 18,is throttled by means of a valve 11. This has the effect that theoperating medium at the outlet of at least one of the evaporating tubesis slightly superheated whereas the other tubes forming the evaporatorgenerally produce wet steam. The temperature at the outlet portion ofthe tube 18 is measured by means of a temperature-sensitive device 12.The pressure in the collector 5 is measured by means of a spring-biasedpiston 13 which adjusts the position of a groove cam 14. A cam followerroller 19 is movable in the groove of the cam 14 and connected to theend of a two-arm lever 15 on the second end of which acts thetemperature-sensitive device 12. The fulcrum of the two-arm lever 15 ismounted to the end of a piston rod connected to the end of a pilot valve16 which controls flow of an actuating fluid to and from a piston 17.The latter is operatively connected to the feed valve 2 for actuatingthe feed valve.

With the aforedescribed arrangement the pressure in the collector 5produces by way of the groove cam 14 a set point for the temperaturewhich is measured by the device 12. If, for example, at unchangedtemperature at the outlet of the tube 18 the pressure in the collectorrises, the piston 13 moves to the right as indicated by arrow +p inFIG. 1. This causes clockwise swinging of the groove cam and movement ofthe roller 19 to the right in FIG. 1. Consequently, the piston valve 16also moves to the right so that pressure fluid can flow (central arrow21) to the left or front side of the piston 17 whereby the feed valve 2is moved in a closing direction. If it is assumed that the pressure inthe collector 5 is not changed and the temperature measured by thedevice 12 rises, the lower end of the lever 15 moves to the left as seenin FIG. 1 (arrow +6). This causes movement of the pilot valve 16 to theleft and flow of pressure fluid to the right of the piston 17 foropening the feed valve 2. It is obvious that in lieu of the single tube18 through which the flow of operating medium is throttled and at theend of which flows slightly superheated steam a plurality of suchreduced-flow tubes may be provided as is shown, for example, in UnitedState Patent No. 2,800,887. In this case a signal corresponding to thetemperatures at the outlet of these tubes may be used for influencingthe feedwater supply to the steam generator.

Without departing from the scope of the present invention the pressuremay be measured at the outlet of an evaporator tube, in the steam spaceof the liquid separator 6 or in the pipe connecting the liquid separatorto the preliminary superheater 7, instead of measuring the pressure inthe collector 5. However, it is advisable to measure the pressure at alocation which is close to the location where the temperature ismeasured.

In the embodiment of the invention shown in FIG. 2, the feedwater passesthrough a feed valve 2 into a preheater 21 and therefrom into anevaporator 4. The effluent of the evaporator flows into a waterseparator 6. The part of the operating medium not removed from thegenerator through the water separator flows consecutively through apreliminary superheater 7, a final superheater and therefrom to a steamconsumer, not shown. A restrictor is arranged downstream of the feedvalve 2 and operatively connected to a device 22 for producing a signalcorresponding to the flow rate of the feedwater. This signal is suppliedthrough a conduit 23 to a proportional-integral-derivative (PID)regulator 24 which produces a set point for a motor operator 25 whichactuates 4. the feed valve 2. The set point of the quick-acting controlcircuit 20, 22, 24, 25, 2 is supplied from a PID regulator 26 whichcompares the actual value of the controlled variable, i.e., thetemperature measured at the outlet of a tube 27 forming part of theevaporator 4 by means of a temperature-sensitive device 12 with a setpoint value produced by a function transmitter 30. If the temperaturesensed by the device 12 is too high the regulator 26 transmits a setpoint signal to the regulator 24 which causes increase of the rate offeedwater supply. The set point signal which is produced by the functiontransmitter 30 is dependent on a signal produced by a device 31 whichsenses the pressure at the outlet of the tube 27. The signal produced bythe device 30 also depends on the temperature at an intermediate pointof one of the tubes of the evaporator 4 measured by a device 32 and onthe temperature of the feedwater entering the evaporator 4 and measuredby a device 33. The device 31 measures the pressure in the tube 27 closeto the location where the temperature is measured by the device 12. Thelocations where temperatures are measured by the devices 32 and 33 areupstream of the location where the operating medium is evaporated.

The temperature of the steam leaving the final superheater 8 which ismeasured by a temperature-sensitive device 40 is controlled in theconventional manner by injecting 'feedwater into the steam passing fromthe preliminary superheater 7 to the final superheater 8.

The operation of the function transmitter 31) will now be described withreference to FIG. 3 which shows another embodiment of the invention. Adevice 12 measures the temperature at the outlet end of evaporator tube29. A spring-biased piston 30" is moved by the temperature-sensitivedevice 12 according to the measured temperature. The piston 30 controlsports for admitting and releasing a pressure fluid to and from acylinder 28 wherein the piston 30" is movable (arrows 30). When thepiston 39" is in rest position the pressure in the cylindercounterbalances the pressure of spring 32' tending to press the piston36 into the cylinder 28. The latter is connected by a pipe 34 to aservomotor cylinder 51) containing a spring-biased piston 35. Theposition of the piston 35 depends on the pressure of the pressure fluidin the cylinders 28 and 50. A cam means or shaped body 51 is connectedto the piston 35 by a rod 36 for axial displacement of the body 51 uponmovement of the piston 35. The pressure of a pressure fluid controlledby the pressure measured by the device 31 (FIG. 2) in the neighborhoodof the separator 6 is conducted to the cylinder 52 through a conduit 53and acts on a springbiased piston 37 reciprocable in the cylinder 52. Achange of the pressure in the conduit 53 causes movement of the piston37 which movement is transmitted to the rod 36 by means of a linkage 38,39 for rotating the body 51. A follower roller 55 is pressed onto thecam surface of the body 51 by means of a spring 45. The roller 55 isoperatively connected to one end of a twoarm lever 56. The fulcrum ofthe lever 56 is formed by the flee end of a piston rod 42 of aservomotor 57 which is hydraulically connected to a temperaturesensitivedevice, for example 32 or 33 in FIG. 2. The second end of the lever 56is connected to a pilot valve 60 which controls supply and release of apressure fluid to and from a servomotor 61. The latter includes a pistonprovided with a piston rod 44 which acts through an angle lever 62 onthe feed valve 2 and which also acts on a fuel control device 65 forcontrolling the fire intensity or rate of fuel supply to the steamgenerator.

Operation of control arrangement according to FIG. 3:

An increase of the temperature in the evaporator tube 29 causes upwardmovement of the piston 30" so that pressure fluid is admitted to thecylinders 28 (upper arrow 30') and 50 and acts on the piston 35. Thelatter is thereby moved to the right in FIG. '3 and moves the body 51also to the right, causing clockwise movement of lever 56 so that pilotvalve 60 is moved downward and pressure fluid (arrows 43) is admitted tothe space in the cylinder 61 above the piston reciprocable therein. Theresult is a downward movement of the piston rod 44 and actuation of thefeed valve 2 in opening direction and of the fuel valve 65 in closingdirection. An increase of the steam pressure in the neighborhood of thewater separator 6 causes upward movement of the piston 37 of theservomotor 52 and revolving of the body 51 in clockwise direction. Theresult is a downward movernent of the roller 55 and counterclockwiseswinging of the lever 56. This causes an upward movement of the pilotvalve 60 and of the piston of the servomotor 61. The feed valve 2 isthereby moved in closing direction and the fuel valve 65 in openingdirection. An increase of the temperature at the location upstream ofthe location where the operating medium is evaporated causes a downwardmovement of the piston of the servomotor 57 and a downward movement ofthe fulcrum of the lever 56 and of the pilot valve 69 and of the pistonof the servomotor 61. This etfects an increase of the rate of feed-Water supply and a reduction of the rate of fuel supply.

In the embodiment of the invention shown in FIG. 2 the functiontransmitter 30 produces a signal depending on the temperatures of theoperating medium at the locations 32, 33 upstream of the evaporation ofthe operating medium and depending on the pressure measured by thedevice 31 in the neighborhood of the water separator 6. This signalforms the set point for the temperature of the slightly superheatedsteam at the outlet of the throttled evaporator tube 2 The arrangementshown in KG. 3 may be interpreted as producing a set point for thetemperature at a location upstream of the evaporation of the operatingfluid by means of the function transmitter 51 in dependence on thetemperature at the outlet of the throttled tube 27 and in dependence onthe pressure measured by the device 31 in the neighborhood of the waterseparator 6.

In the method and apparatus according to the invention, of threevariables, namely the temperature at the outlet of a throttledevaporator tube, the pressure in the neighborhood of a water separatorarranged immediately downstream of the evaporator, and the temperatureupstream of the zone where evaporation begins in the evaporator, twovariables are used for producing a set point signal for controlling thethird variable.

I claim:

1. A method of influencing the rate of feedwater supply to a forced flowsteam generator including means for reguiating the rate of feedwatersupply to the steam generator, an evaporator composed of a plurality oftubes arranged in parallel relation with respect to the flow ofoperating medium therethrough, one of said tubes being provided with athrottling means and being operated to produce slightly superheatedsteam, the others of said tubes being operated to produce substantiallysaturated steam, and a water separator arranged downstream of saidevaporator, the method comprising:

measuring the temperature of the steam substantially at the outlet ofsaid tube producing slightly superheated steam for producing a signalcorresponding to said temperature,

measuring the pressure of the steam adjacent said water separator forproducing a signal corresponding to said pressure,

combining said two signals for producing a combined signal, and

influencing said means for regulating the rate of feedwater supply bysaid combined signal for increasing the rate of feedwater supply upon anincrease of the temperature and for decreasing the rate of feed Watersupply upon an increase of the pressure, and conersely.

2. A method as defined in claim 1 wherein said two signals are combinedin predetermined proportions.

3. A method of influencing the rate of feedwater sup ply to a forcedflow steam generator including means for regulating the rate offeedwater supply to the steam generator, an evaporator composed of aplurality of tubes arranged in parallel relation with respect to theflow of operating medium therethrough, one of said tubes being providedwith a throttling means and being operated to produce slightlysuperheated steam, the others of said tubes being operated to producesubstantially saturated steam, and a water separator arranged downstreamof said evaporator, the method comprising:

measuring the temperature of the steam substantially at the outlet ofsaid tube producing slightly super heated steam for producing a signalcorresponding to said temperature,

measuring the pressure of the steam adjacent said water separator forproducing a signal corresponding to said pressure,

measuring the temperature in said evaporator upstream,

with respect to the flow of the operating medium, of the location whereevaporation of the operating medium begins, for producing an additionaltemperature-responsive signal,

combining said three signals for producing a combined signal, and

influencing said means for regulating the rate of feedwater supply bysaid combined signal for increasing the rate of feedwater supply upon anincrease of the first mentioned temperature and of the additionaltemperature and for decreasing the rate of feedwater supply upon anincrease of the pressure, and conversely.

4.. In combination with a forced flow steam generator having anevaporator, means for supplying feedwater thereto, said evaporatorcomprising a plurality of tubes arranged in parallel relation withrespect to the flow of operating medium therethrough, one of said tubeshaving an outlet portion and a throttling means for delivering slightlysuperheated steam through said outlet portion, the others of said tubesdelivering substantially saturated steam, and conduit means connected tosaid evaporator for receiving operating medium therefrom, said conduitmeans including a Water separator:

temperature-sensitive means connected to said outlet portion forproducing a signal corresponding to the temperature of the steamdelivered through said outlet portion,

pressure-sensitive means connected to said conduit means for producing asignal corresponding to the pressure of the steam adjacent saidseparator, and

means for combining said two signals for producing a combined signal,said feedwater supply means including means for influencing the rate offeedwater supply to said evaporator,

said last mentioned means being operatively connected to said signalcombining means for receiving the combined signal therefrom and foractuation by said combined signal, for increasing the rate of feedwatersupply upon an increase of the temperature and for decreasing the rateof feedwater supply upon an increase of the pressure, and conversely.

5. In the combination defined in claim 4, wherein said signal combiningmeans includes means for combining said signals in predeterminedproportions.

6. In the combination defined in claim 4, wherein said signal combiningmeans is in the form of a proportional, integral, derivative regulator.

'7. In combination with a forced flow steam generator having anevaporator, means for supplying feedwater thereto, said evaporatorcomprising a plurality of tubes arranged in parallel relation withrespect to the flow of operating medium therethrough, one of said tubeshaving an outlet portion and a throttling means for delivering slightlysuperheated steam through said outlet portion,

the others of said tubes delivering substantially saturated steam, andconduit means connected to said evaporator for receiving operatingmedium therefrom, said conduit means including a Water separator:

temperature-sensitive means connected to said outlet portion forproducing a signal corresponding to the temperature of the steamdelivered through said outlet portion,

pressure-sensitive means connected to said conduit means for producing asignal corresponding to the pressure of the steam adjacent saidseparator,

second temperature-sensitive means connected to said evaporatorupstream, with respect to the flow of the operating medium, of thelocation Where evaporation of the operating medium begins, for producingan additional temperature-corresponding signal, and

means for combining said three signals for producing a combined signal,

said feedwater supply means including means for infiuencing the rate offeedwater supply to said evaporator,

said last mentioned means being operatively connected to said signalcombining means for receiving the combined signal therefrom and foractuation by said combined signal, for increasing the rate of feedwatersupply upon an increase of the first mentioned temperature and of theadditional temperature and for decreasing the rate of feedwater supplyupon an increasing of the pressure, and conversely.

8. In combination with a forced flow steam generator having anevaporator, means for supplying feedWater thereto, said evaporatorcomprising a plurality of tubes arranged in parallel relation withrespect to the flow of operating medium therethrou-gh, one of said tubeshaving an outlet portion and a throttling means for delivering slightlysuperheated steam through said outlet portion, the others of said tubesdelivering substantially saturated steam, and conduit means connected tosaid evaporator for receiving operating medium therefrom, said conduitmeans including a Water separator:

temperature-sensitive means connected to said outlet portion forproducing a signal corresponding to the temperature of the steamdelivered through said outlet portion,

pressure-sensitive means connected to said conduit means for producing asignal corresponding to the pressure of the steam adjacent saidseparator,

second temperature-sensitive means connected to said evaporatorupstream, with respect to the flow of the operating medium, of thelocation Where evaporation of the operating medium begins, for producingan additional temperaturecorresponding signal,

a function transmitter operatively connected to said secondtemperature-sensitive means and to said pressure-sensitive means forreceiving signals therefrom and producing a combined signal, and

means for combining said combined signal with said firsttemperature-corresponding signal for producing a set point signal,

said feedwater supply means including means for measuring the rate offlow of feedwater, and means for influencing the rate of feedwatersupply to said evaporator,

said means for influencing the rate of feedwater supply to the steamgenerator being connected to said measuring means and to said lastmentioned combining means for receiving said set point signal therefromand for increasing the rate of feedwater supply upon an increase of saidset point signal, and conversely.

9. In combination with .a forced flow steam generator having anevaporator, means for supplying feedwater thereto, said evaporatorcomprising a plurality of tubes arranged in parallel relation withrespect to the flow of operating medium therethrough, one of said tubeshaving an outlet portion and a throttling means for delivering slightlysuperheated steam through said outlet portion, the others of said tubesdelivering substantially saturated steam, and conduit means connected tosaid evaporator for receiving operating medium therefrom, said conduitmeans including a Water separator:

temperature-sensitive means connected to said outlet portion forproducing a signal corresponding to the temperature of the steamdelivered through said outlet portion,

pressure-sensitive means connected to said conduit means for producing asignal corresponding to the pressure of the steam adjacent saidseparator,

second temperature-sensitive means connected to said evaporatorupstream, with respect to the flow of the operating medium, of thelocation Where evaporation of the operating medium begins, for producingan additional temperature-corresponding signal,

a function transmitter operatively connected to said secondtemperature-sensitive means and to said pressure-sensitive means forreceiving signals therefrom and producing a combined signal, and

means for combining said combined signal with said firsttemperature-corresponding signal for producing a set point signal,

said feedwater supply means including means for measuring the rate offlow of feedwater, means for influencing the rate of feedwater supply tosaid evaporator, a motor operator for said rate of feedwater supplyinfluencing means, and a regulator for said motor operator,

said regulator being connected to said measuring means and to saidcombining means for receiving said set point signal therefrom and foractuation by said set point signal for increasing the rate of feedwatersupply upon an increase of said set point signal, and conversely.

It). In combination With a forced flow steam generator having anevaporator, means for supplying feedwater thereto, said evaporatorcomprising a plurality of tubes arranged in parallel relation withrespect to the flow of operating medium therethrough, one of said tubeshaving an outlet portion and a throttling means for delivering slightlysuperheated steam through said outlet portion, the others of said tubesdelivering substantially saturated steam, and conduit means connected tosaid evaporator for receiving operating medium therefrom, said conduitmeans including a Water separator:

temperature-sensitive means connected to said outlet portion forproducing a signal correspondingto the temperature of the steamdelivered through said outlet portion,

pressure-sensitive means connected to said conduit means for producing asignal corresponding to the pressure of the steam adjacent saidseparator,

sec oud-temperature-sensitive means connected to said evaporator at apoint upstream of the location where evaporation of the operating mediumbegins for producing a signal corresponding to the temperature of theoperating medium before evaporation thereof begins,

a function transmitter operatively connected to said pressure-sensitivemeans and to said first temperature-sensitive means for receiving therespective signals and for producing a signal therefrom, and

means connected to said function transmitter and to said secondtemperature-sensitive means for combining the signal produced by saidsecond temperature-sensitive means with the signal produced by saidfunction transmitter for producing a set point signal,

said feedwater supply means including means for measuring the rate offlow of feedwater and means for influencing the rate of feedWater supplyto said ev p rator,

said last mentioned means including a motor operator and a regulatortherefor,

said regulator being operatively connected to said measuring means andto said combining means for receiving said set point signal therefromand for actuation by said set point signal for increasing the feedwatersupply upon an increase of said set point signal, and conversely.

References Cited by the Examiner UNITED STATES PATENTS Junkins 122-451 XHolle 122448 Profos 122-451 Schroedter et al. 12245 1.1

CHARLES J. MYHRE, Primary Examiner.

1. A METHOD OF INFLUENCING THE RATE OF FEEDWATER SUPPLY TO A FORCED FLOWSTEAM GENERATOR INCLUDING MEANS FOR REGULATING THE RATE OF FEEDWATERSUPPLY TO THE STEAM GENERATOR, AN EVAPORATOR COMPOSED OF A PLURALITY OFTUBES ARRANGED IN PARALLEL RELATION WITH RESPECT TO THE FLOW OFOPERATING MEDIUM THERETHROUGH, ONE OF SAID TUBES BEING PROVIDED WITH ATHROTTLING MEANS AND BEING OPERATED TO PRODUCE SLIGHTLY SUPERHEATEDSTEAM, THE OTHERS OF SAID TUBES BEING OPERATED TO PRODUCE SUBSTANTIALLYSATURATED STEAM, AND A WATER SEPARATOR ARRANGED DOWNSTREAM OF SAIDEVAPORATOR, THE METHOD COMPRISING: MEASURING THE TEMPERATURE OF THESTEAM SUBSTANTIALLY AT THE OUTLET OF SAID TUBE PRODUCING SLIGHTLYSUPERHEATED STEAM FOR PRODUCING A SIGNAL CORRESPONDING TO SAIDTEMPERATURE, MEASURING THE PRESSURE OF THE STEAM ADJACENT SAID WATERSEPARATOR FOR PRODUCING A SIGNAL CORRESPONDING TO SAID PRESSURE,COMBINING SAID TWO SIGNALS FOR PRODUCING A COMBINED SIGNAL, ANDINFLUENCING SAID MEANS FOR REGULATING THE RATE OF FEEDWATER SUPPLY BYSAID COMBINED SIGNAL FOR INCREASING THE RATE OF FEEDWATER SUPPLY UPON ANINCREASE OF THE TEMPERATURE AND FOR DECREASING THE RATE OF FEED WATERSUPPLY UPON AN INCREASE OF THE PRESSURE, AND CONVERSELY.